British Suffix -io

Dear members!

Writing my diploma thesis on Latin place-names in Britain I came
across the Celtic suffix -io.
It is used as a derivational suffix and can either be added to
adjectives
(cf. the place-name of Coccio < brit.*cocco- 'red' + *-io, now
Edgeworth, Lancashire)
or
to nouns (cf. the place-name of Gobannio < brit. *gobannia 'river of
the blacksmiths' + *io, now Abergavenny, Monmouthshire).

Unfortunately, I could neither find out anything about the meaning
nor about the origin of the suffix.

I would be extremely grateful for any help or advice.

With kindest regards,

Peter Ehlers